Interpretive Summary: A thermal-based land surface modeling scheme is described which computes heat flux and evaporation from the soil and vegetation components using a unique “Two-Source Model” (TSM) technique with remotely sensed surface temperature. This modeling approach is a major advancement in the application of remotely sensed surface temperature for mapping evpotranspiration (ET) over many prior approaches using “One-Source Model” (OSM) methodologies that are problematic when applied to heterogeneous landscapes. The capability of TSM to reliably compute heat and evaporative fluxes from soil and vegetation components over a wide range of environmental conditions is tested using simulated data from a complex Soil-Vegetation-Atmosphere-Transfer (SVAT) model Cupid that simulates the complete radiation, heat and evaporation processes occurring at the soil/canopy interface. The results show that the TSM scheme outperformed the OSM approaches and that by computing the fluxes from the soil and vegetation, this provided information critical for assessing vegetation water use and stress as well as information concerning the vertical distribution of available water in surface layer and root zone. Mapping of ET for actual landscapes is presented using the TSM as the land surface scheme coupled to a regional energy balance model, the Atmosphere Land EXchange Inverse/Disaggregation ALEXI ( ALEXI/DisALEXI ) model. ALEXI is designed for operational applications at local to continental scales using multi-scale thermal imagery. This modeling strategy is shown to be viable approach for operationally monitoring water use, crop stress, and drought at regional scales, information that is essential for addressing climatic and environmental impacts on agricultural production.

Technical Abstract:
Over 10 years ago, John Norman and co-authors proposed a thermal-based land surface modeling strategy that treated the energy exchange and kinetic temperatures of the soil and vegetated components in a unique “Two-Source Model” (TSM) approach. The TSM formulation addresses key factors affecting the convective and radiative exchange within the soil-canopy-atmosphere system, focusing on the relationship between radiometric and aerodynamic temperature. John Norman’s contribution came at a time when thermal-based techniques applied to standard “One-Source Model” (OSM) for large scale land surface flux and evapotranspiration (ET) estimation was generally considered unreliable and not viable for operational remote sensing applications. Others have subsequently modified OSM schemes to accommodate the radiometric-aerodynamic temperature relationship for partial canopy cover conditions, approaching accuracies achieved with the TSM. In this study, a range in canopy cover and moisture conditions simulated by the detailed Soil-Vegetation-Atmosphere-Transfer (SVAT) model Cupid developed by John Norman, which simulates the complete radiation, convection/turbulence and hydrologic processes occurring at the soil/canopy interface, is used to evaluate Norman’s TSM and two current OSM schemes. The utility of the TSM versus OSM approaches in handling extremes in moisture/vegetation cover conditions simulated by the SVAT model Cupid is presented. Generally the TSM approach outperformed the OSM schemes for the extreme conditions. Moreover, the ability of the TSM to partition ET into evaporation and transpiration components provides additional hydrologic information about the moisture status of the soil and canopy system, and about the vertical distribution of moisture in the soil profile (surface layer vs. root zone). Examples for actual landscapes are presented in the application of the TSM as incorporated within in the Atmosphere Land EXchange Inverse/Disaggregation ALEXI ( ALEXI/DisALEXI ) modeling system, designed for operational applications at local to continental scales using multi-scale thermal imagery. This strategy for utilizing radiometric surface temperature in land surface modeling has converted many skeptics and more importantly rejuvenated many in the research and operational remote sensing community to reconsider the utility of thermal infrared remote sensing for monitoring land surface fluxes from local to regional scales.